1. Field of the Invention
The present invention relates to an automotive electric heater apparatus utilizing a high voltage power supply of 200 V to 400 V DC.
2. Description of the Related Art
Automotive vehicles such as a fuel cell-powered vehicle, an electric car and a hybrid car with little waste heat are designed to employ an electric heater as a heating system by using a high-voltage power supply together with a thermal fuse for protecting the electric heater (Japanese Unexamined Patent Publication No. 2002-324653).
The thermal fuse, as shown in FIGS. 4A, 4B, includes a temperature detecting element 120 (low-melting alloy) connected to an electric heater 110 through lead wires 100. A ceramic container 130 accommodating the temperature detecting element 120 is inserted into an insulating tube 140 together with the lead wires 100, and the insulating tube 140 is mounted in contact with an aluminum body 150 of the electric heater 110. In the case where the electric heater 110 is abnormally overheated, heat is transmitted from the aluminum body 150 to the lead wires 100 in the insulating tube 140, and when the temperature detecting element 120 which has received the heat from the lead wires 100 reaches a set temperature, the thermal fuse fuses and turns off the power circuit of the electric heater 110.
In the thermal fuse described above, however, only a peripheral part of the insulating tube 140 is in contact with the surface of the aluminum body 150 of the electric heater 110 as shown in FIG. 4B and, therefore, the contact area between the insulating tube 140 and the aluminum body 150 is very small. Also, an air layer (an air layer in the insulating tube 140) low in heat conductivity in the path of the heat transmitted from the aluminum body 150 to the lead wires 100 causes a considerable length of time to be consumed before the temperature detecting element 120 blows (the thermal fuse has a low temperature sensitivity). As a result, the surface temperature of the aluminum body 150 increases greatly (to, say, 400° C. or higher) before the temperature detecting element 120 actually fuses, thereby posing a safety problem.
Also, in the thermal fuse shown in FIGS. 4A, 4B, the two lead wires 100 connected to the temperature transducer 120 are led out in opposite directions. Therefore, a harness (not shown) connected to one of the lead wires 100 is required to be laid in U-turn form, and the resulting detour-connection of harness reduces the working efficiency.